Formal sponsorship letters in Germany are called Verpflichtungserklärung. They mean that the sponsor is posting a bond and they are only used if the visa would otherwise be denied. If you did not need a Verpflichtungserklärung to get your visa, you probably will not need one now.
Your visa had both a duration of stay and a validity period. You can stay at most the number of days indicated in the duration, but only within the validity period, whichever is less. Usually the validity period is longer than the duration so that the visitor can adjust the travel date a little. Are you still within the validity period?
Another thing is that you should still be traveling for the purpose that was given in your application. Are you still traveling for the same purpose?
If the answer to both questions is yes, you can travel with the old visa.